Obsessive-compulsive disorder (OCD) is a common and disabling disorder that was until recently relatively unstudied, but is currently the focus of considerable research into the treatment and pathophysiology. Broad training is proposed in three phases. The first involves core studies of the pathophysiology of OCD, including pharmacological challenges, neuropsychiatric studies, electrophysiology, peripheral and CSF markers, and diagnostic assessment. These core studies will be performed at baseline and again during treatment with serotonin reuptake inhibitors. The second phase involves development of new studies in OCD, such as imaging (RCBF, SPECT), additional biological challenges to clarify issues raided in phase I, evoked potentials pre- and post challenges, and epidemiologic, pedigree and DNA marker studies of OCD. The third phase involves comparison of the core findings in OCD with related disorders. These include comparison with normal controls, other anxiety controls, such as social phobia and panic disorder, high risk relatives, patients with concurrent OCD and major depression (a common occurrence), childhood onset OCD, and genetically linked disorders such as Tourette's syndrome. Clinical issues such as the prediction of relapse, and the prediction of development in high risk relatives, will be addressed. In the course of these studies, broad training with recognized experts will be received in several areas, including pharmacological challenges, neurochemistry, neuropsychology, imaging, epidemiology, electrophysiology, Tourette's syndrome, childhood onset OCD, psychopharmacology, and statistics and research design. This training will develop further expertise, and permit further studies of the pathophysiology of related disorders.